1. Field of the Invention
This invention relates to metallic powders and, in particular, to the compaction of such powders to form metallic parts using powder metallurgy. However, this invention is not limited to the powder metallurgy field and can be applied in the pharmaceutical field for instance or any other fields requiring the lubrication of a die cavity prior to shaping.
2. Brief Description of the Background Art
In powder metallurgy ("P/M"), metal powders are compacted in a die cavity to form a green compact which is then heat treated or sintered at relatively high temperatures to create metallic bonds between particles to form a metallic part. During compaction, friction is generated between the metal powder particles themselves and also between the metal powder particles and the die wall, causing both adhesive wear on the die surfaces and lamination or breakage of the green compact after ejection from the die cavity. In order to decrease the friction between the metal powder particles and the die walls and to decrease the ejection force required to eject the green compact from the die cavity, dry lubricants have been historically added to the metal powder mixture. These are generally referred to as internal lubricants since they are admixed with the metal powder to be compacted.
It is well known that wet lubricants promote clumping of the metal powder and adversely affect the flow characteristics of P/M materials, and then they cannot be used successfully. On the other hand, dry lubricants have been used successfully since they are non-binding and do not affect flow characteristics. Due to the pressures and temperatures involved during compaction, dry lubricants typically melt and flow between the metal powder particles and lubricate the die walls. However, one disadvantage of using a dry lubricant in the metal powder formulation is that both the final density and the strength of the metallic part are less than theoretically achievable when no lubricant is admixed. In fact, the density of common lubricants used is usually lower than the density of the metal powders used.
Prior attempts at eliminating the addition of internal lubricants in the metal powder composition focused on spraying onto the die walls liquid lubricants, or dry lubricants that were dispersed in solvents. However, the poor distribution of liquid applied to the die walls limited the size and the shape of the green compact. Moreover, the use of dispersed dry lubricants poses numerous health, safety and environmental hazards due to the presence of volatile solvents.
Up to now, only a few systems have been developed to apply dry lubricants to die cavity walls. One system described in the prior art uses a tribogun to spray, directly from the outside of the die cavity, an electrostatically charged lubricant into the die cavity. Although this technique is simple, it can only be used for small dies and does not achieve uniform distribution of the lubricant in the die cavity. In another device, such as the one described in U.S. Pat. No. 4,840,052, a fluid mixture consisting of a lubricant and compressed air is used to lubricate the surfaces of die punches of a forging press before the part is made. In this case the lubricant coating applied with this device is localized and non-uniform. Another example of a device used to apply lubricant is the one described in U.S. Pat. No. 5,642,637 which is dedicated to die forging. In this case, the forging cavity was not coated and the lubrication, as in U.S. Pat. No. 4,840,052, was limited to the punch surfaces. Moreover, in this patent, the surfaces to be lubricated are not located in a die cavity.
The objective of the present invention is to overcome drawbacks and disadvantages of the prior art, and to provide an improved method of applying dry lubricant to die cavity walls in order to improve the manufacture of metallic parts by powder metallurgy. The apparatus of the present invention was developed to apply a constant, thin and uniform dry lubricant to the die cavity walls to make improved quality powder metallurgy parts.